Adsorption of CO, O2, NO2, and NH3 by Metallophthalocyanine

Ordered monolayers of metal-free (H2Pc), magnesium (MgPc), chloroaluminum (AlClPc), vanadyl (VOPc), manganese (MnPc), iron (FePc), cobalt (CoP), nicke...
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Langmuir 1997, 13, 5184-5188

Adsorption of CO, O2, NO2, and NH3 by Metallophthalocyanine Monolayers Supported on Graphite K. Morishige,* S. Tomoyasu, and G. Iwano Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700, Japan Received March 17, 1997. In Final Form: June 30, 1997X Ordered monolayers of metal-free (H2Pc), magnesium (MgPc), chloroaluminum (AlClPc), vanadyl (VOPc), manganese (MnPc), iron (FePc), cobalt (CoP), nickel (NiPc), copper (CuPc), zinc (ZnPc), silver (AgPc), tin (SnPc), and lead phthalocyanines (PbPc) were prepared by equilibrium adsorption from gas phase onto graphite at high temperatures, and gas adsorptions of CO, O2, NO2, and NH3 by these monolayer films were examined. In the 13 kinds of phthalocyanines examined here, only the FePc monolayer shows appreciable adsorptive capacity of CO at 273 K. Two types of O2 adsorptions, with smaller and larger activation energy, occur on the MnPc. At 573 K, O2 adsorption on the MnPc, FePc, and SnPc monolayers results in formation of stoichiometric compounds of composition MPc‚O2 on graphite. A comparison between adsorptive capacity and the number of the MPc molecules constituting a monolayer shows that NO2 is strongly bound to the metal centers of the MnPc, FePc, CoPc, and SnPc monolayers to form stoichiometric compounds of composition MPc‚NO2 on graphite and NH3 forms stoichiometric compounds of composition MPc‚NH3 with the MgPc, MnPc, FePc, CoPc, and ZnPc monolayers at 273 K.

1. Introduction Gas adsorption on metallophthalocyanines (MPcs) has been extensively investigated,1 mainly in relation to their gas-sensing capability and catalysis. All of these measurements, however, have been carried out on MPcs in the form of thin films or powders. Morphology of thin films and powders is complex, and their surfaces will present a range of different adsorption sites corresponding to different crystal faces, edges, and corners as well as a range of structural defect sites.1-3 Moreover, for the large planar MPc molecules, the extent to which the surface of each molecule is available to adsorbing gas depends upon the sites. For example, a molecule within a stack presents mainly its edges as adsorption sites, with adsorption near the center of the molecule effectively blocked by the other neighbors within the stack, whereas a molecule on the end of a stack presents its entire surface toward any approaching gas molecules. Such surface heterogeneity effects on adsorption are further complicated by the possibility of gas penetration into the lattice and surface restructuring accompanying adsorption. Therefore, interpretation of adsorption data obtained on the microcrystalline forms of MPcs is not straightforward. Very recently, scanning tunneling microscope investigations4 showed that copper phthalocyanine (CuPc) forms a well-ordered monolayer of the flat lying molecules on the basal plane of highly oriented pyrolytic graphite. The complexity of the situation is expected to be considerably diminished in the case of gas adsorption by such monolayer films of MPcs. The MPc molecules at the surface will have the same local environments, and the metal centers of all the MPc molecules constituting the monolayers are accessible to incoming reactant. Since the catalytic and electrocatalytic activities of transition metal macrocycles are most often associated with the axial coordination of the reactant to the metal center, such monolayers may be expected to show high activities toward the catalytic reactions in which MPcs are effective. The purpose of the X Abstract published in Advance ACS Abstracts, September 1, 1997.

(1) Wright, J. D. Prog. Surf. Sci. 1989, 31, 1. (2) Contour, J. P.; Lenfant, P.; Vijh, A. K. J. Catal. 1973, 29, 8. (3) Sappok, R. J. Oil Colour Chem. Assoc. 1978, 61, 299. (4) Ludwig, C.; Strohmaier, R.; Petersen, J.; Gompf, B.; Eisenmenger, W. J. Vac. Sci. Technol., B 1994, 12, 1963.

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present study is to prepare the monolayer of MPcs on exfoliated graphite and to examine the correlation between gas adsorption and the kind of the central metals. 2. Experimental Section 2.1. Materials. The substrate used was recompressed exfoliated graphite, Grafoil.5 It has a specific surface area of 16.8 m2/g, which was determined from the adsorbed amount of the commensurate-incommensurate transition of Kr at 90 K by assuming the cross section of Kr to be 0.1572 nm2. The metal-free (H2Pc), magnesium (MgPc), chloroaluminum (AlClPc), iron (FePc), cobalt (CoPc), nickel (NiPc), copper (CuPc), zinc (ZnPc), silver (AgPc), tin (SnPc) and lead phthalocyanines (PbPc) were supplied by Tokyo Chemical Industry Co., Ltd. Vanadyl (VOPc) and manganese phthalocyanines (MnPc) were synthesized according to published procedures.6 Except for AgPc, they were all purified by sublimation at 723 K in vacuo. AgPc was used without further purification. Carbon monoxide, oxygen, nitrogen dioxide, and ammonia supplied from Takachiho Chemical Industry Co., Ltd., were used as adsorbates without further purification. 2.2. Preparation of MPc Monolayers. The monolayers were prepared by equilibrium adsorption of MPcs from gas phase onto graphite. Grafoil (0.5 g) in a Pyrex tube was evacuated for 5 h at 723 K, and then a small amount of MPc (usually 10 mg) was added at room temperature. After the mixture were evacuated for 1 h at room temperature, the Pyrex tube was sealed by glass blowing. In order to prepare the ordered monolayers of MPcs on the substrate, the mixtures were heated for 12 h at a particular temperature (usually 723 K), where the vapor pressures of the MPcs seem to become significant. 2.3. Measurements. The X-ray diffraction patterns of the substrates both with and without the monolayers were measured at room temperature with Cu KR radiation in a symmetrical transmission geometry in air.5 The adsorption isotherms of gases were measured volumetrically on 0.5 g of Grafoil modified with the monolayers of MPcs by using an apparatus equipped with a Baratron (5) Morishige, K.; Kawamura, K.; Yamamoto, M.; Ohfuji, I. Langmuir 1990, 6, 1417. (6) Barrett, P. A.; Dent, C. E.; Linstead, R. P. J. Chem. Soc. 1936, 1719.

© 1997 American Chemical Society

Gas Adsorption on MPc

Figure 1. X-ray diffraction patterns of metal-free and 3d metal phthalocyanine monolayers on graphite.

capacitance manometer (type 310-BH) with a full scale of 100 Torr. 3. Results and Discussion 3.1. Structure of Monolayer. As is usual in diffraction studies of overlayers, all the diffraction patterns presented in this study are those with background subtracted. Figure 1 shows the X-ray diffraction patterns of monolayer films of H2Pc and 3d metal (Mn, Fe, Co, Ni, Cu, Zn) Pcs supported on graphite, where these monolayer films were prepared by equilibrium adsorption of the corresponding MPcs from gas phase onto graphite at 723 K. The features around 2θ ) 26.5°are due to the incomplete subtraction of the strong graphite (002) reflection. The patterns, except for that of MnPc, resemble each other and show several sharp diffraction peaks, which suggest the formation of an ordered monolayer of the molecules on graphite. The effect of the preparation temperature (the adsorption temperature of H2Pc and MPcs on graphite) on X-ray diffraction patterns was

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examined at four different temperatures of 673, 693, 723, and 753 K. Lower preparation temperatures tend to result in the appearance of a slightly diffuse pattern, although the diffraction patterns did not appreciably depend on these preparation temperatures. This probably reflects the lowering in diffusion constants of the molecules on graphite surfaces. Very recently, Ludwig et al.4 observed scanning tunneling microscopy (STM) images of CuPc molecules on highly oriented pyrolytic graphite (HOPG) prepared by organic molecular beam epitaxy and showed that on HOPG the flat lying molecules form a close-packed structure with a nearly square unit cell (z ) 1, a ) 1.37 nm, b ) 1.36 nm, and γ ) 90.3°). In this monolayer, the molecule occupies the area of 1.86 nm2 on graphite surfaces. The structure belongs to the plane group symmetry p2. As Figure 2 shows, this model was able to reproduce satisfactorily the observed diffraction pattern of the CuPc monolayer prepared from the equilibrium adsorption method. The molecular structure was adopted from the single crystal data of CuPc.7 Only the C, N, and Cu atoms were included in the calculation. The diffraction pattern was calculated according to the powder-averaged Gaussian line shape appropriate to a crystalline monolayer, and the profile parameters were used as determined previously in this substrate.5 The calculated pattern is shown as a solid line in Figure 2. The quality of the fit is reasonable. Similarly only slight adjustment of the lattice parameters mentioned above gave reasonable fits between the observed and calculated diffraction patterns for the monolayers of H2Pc, FePc, CoPc, NiPc, and ZnPc. Although most of Mpcs are thermally very stable, thermal decomposition of MnPc may occur to some extent during equilibrium adsorption at such high temperatures. In these monolayers, the MPc molecules at the surface have the same local environments and the metal centers of all the molecules constituting a monolayer are accessible to any approaching gas molecules. As Figure 3 shows, the diffraction patterns of the MgPc, AlClPc, VOPc, AgPc, and PbPc monolayers have several sharp peaks for each of them, while that of SnPc is rather diffuse. Thermal decomposition might occur for SnPc. Although ordered monolayers were formed for MgPc, AlClPc, VOPc, AgPc, and PbPc on graphite, the STM model of Ludwig et al. did not give reasonable fits between the observed and calculated diffraction patterns for these monolayers. The structures of these monolayers remain to be solved.

Figure 2. X-ray diffraction pattern of the CuPc monolayer on graphite. Solid line denotes the calculated diffraction pattern as described in the text.

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Morishige et al.

Figure 5. Saturated amounts of CO adsorbed reversibly and the amounts of NO2 adsorbed irreversibly on the FePc monolayer at 273 K as a function of loading amount of FePc.

Figure 3. X-ray diffraction patterns of metallophthalocyanine monolayers on graphite.

Figure 4. Adsorption isotherms of CO on the FePc monolayer at 273 K as a function of pretreatment temperature.

3.2. CO Adsorption. In the 13 kinds of phthalocyanines examined here, only the FePc monolayer showed appreciable adsorptive capacity of carbon monoxide at 273 K. First, the FePc monolayer supported on exfoliated graphite was prepared from heating the mixtures of graphite (0.5 g) and FePc (10 mg) in a sealed glass ampule at 723 K for 12 h, transferred into adsorption cell at room temperature in air, and then pretreated successively at higher temperatures for 5 h in vacuo. An adsorption isotherm of CO was measured as a function of pretreatment temperature. As Figure 4 shows, the amount of adsorption increased with increasing pretreatment temperature. This indicates that the adsorption sites of CO are covered with impurities, perhaps O2 and H2O from (7) Brown, C. J. J. Chem. Soc. A 1968, 2488.

air. Next, in order to examine the effect of the preparation temperature of the monolayer on CO adsorption, the FePc monolayers were prepared from heating the mixture of graphite and FePc at various temperatures between 673 and 753 K. However, it was found that the adsorption isotherms of CO were almost independent of their preparation temperatures. Finally, the effect of the loading amount of FePc on the adsorption isotherms of CO was examined. Figure 5 shows the relationship between the loading amount of FePc and the saturated amount of adsorption of CO, where the saturated amount of adsorption was determined from applying the Langmuir equation into the adsorption isotherms. In this figure, the relationship between the loading amount of FePc and the amount of irreversible adsorption of NO2 at 273 K is also illustrated, although the explanation for this experiment will be presented in a later section. When the loading amount of FePc against 1.0 g of graphite is suppressed below 2 × 10-5 mol, the amounts of adsorption of CO and NO2 are almost proportional to the loading amount of FePc. This indicates that the loading amount of 2 ×10-5 mol against 1.0 g of graphite corresponds to completion of a monolayer of FePc on graphite. When we consider the surface area of graphite (18.6 m2/g), the occupied area of the FePc molecule on graphite is estimated to be ∼1.4 nm2, being approximately consistent with the sectional area of the molecule (1.86 nm2) from the crystallographic data mentioned above. The ratio of the amount of adsorption to the loading amount is roughly 1:2. This ratio is not necessarily consistent with the simple idea that CO molecules adsorb onto the metal centers of FePc. The adsorbed CO molecules desorbed easily on evacuation at room temperature. Unless otherwise stated, the loading amount of 10 mg against 0.5 g of Grafoil, the preparation temperature of 723 K, and the pretreatment temperature of 623 K were employed in the following adsorption experiments. 3.3. O2 Adsorption. Figure 6 shows the temperature dependence of O2 adsorption onto the MnPc and FePc monolayers. On the FePc, the amount of adsorption increases with increasing temperature between 273 and 573 K, which indicates the occurrence of activated adsorption of O2 on the monolayer. In addition to the occurrence of the activated adsorption, the MnPc monolayer shows appreciable adsorptive capacity of O2 at the low temperatures of 195 and 273 K. This indicates the occurrence of two types of adsorptions of O2 on the MnPc monolayer, namely, with smaller and larger activation energy. Table 1 summarizes the saturated amounts of O2 adsorbed at 573 K on the MPc monolayers, estimated from the Langmuir plot. Assuming that the monolayer capacity of MPc molecules on Grafoil graphite is 2 ×10-5 mol/g, the

Gas Adsorption on MPc

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Figure 6. Adsorption isotherms of O2 on the MnPc and FePc monolayers as a function of temperature. Table 1. Saturated Amounts of O2 Adsorbed on the MPc Monolayers at 573 K central metal

V (mL (STP)/g)

central metal

V (mL (STP)/g)

H2 Mg AlCl VO Mn Fe Co

0.17 0.13 0.02 0.03 0.46 0.54 0.02

Ni Cu Zn Ag Sn Pb

0.05 0.03 0.03 0.17 0.46 0.10

amount of gas molecules needed to form a stoichiometric compound of MPc‚L (L, gas molecules) on graphite becomes 0.45 mL (STP)/g. This value is approximately consistent with the saturated amounts of O2 adsorption on the MnPc, FePc, and SnPc at 573 K. At 273 K, only the MnPc showed appreciable adsorptive capacity of O2. Previous studies concerning O2 adsorption on MPcs are conflicting. Auger spectroscopy8 and surface photovoltage studies9 have shown that a monolayer of O2 is easily formed on CoPc and NiPc by exposure to O2 of low pressure at room temperature. On the other hand, thermal desorption spectroscopy studies2,10 have shown that the adsorption of O2 strongly depends on the adsorption temperature, and the amounts of O2 adsorbed on H2Pc, FePc, CoPc, and CuPc at room temperature are all very small compared to a monolayer capacity. Our results are consistent with the thermal desorption spectroscopy experiment.10 Both Auger spectroscopy and surface photovoltage measurements utilize electron beams. Irradiation with electron beams might cause degradation of MPc molecules and subsequent incorporation of O2 into the MPc molecules. 3.4. NO2 Adsorption. After evacuation of the MPc monolayers at 623 K, an adsorption isotherm of NO2 was measured at 273 K (the first adsorption isotherm). After each adsorption run the monolayers were evacuated at room temperature for 4 h in a vacuum of 1 × 10-5 Torr and the adsorption was remeasured (the second adsorption isotherm). The difference in amount of adsorption between the first and second adsorption isotherms represents the amount of irreversibly adsorbed species. The adsorption isotherms can be classified into three classes according to the extent of the irreversible adsorption. Typical examples are illustrated in Figure 7. The extent of the irreversible adsorption of NO2 is large on the monolayers of MnPc, FePc, CoPc, and SnPc, while H2Pc, MgPc, AlClPc, and ZnPc adsorb only a small amount of NO2 irreversibly. The cases of VOPc, NiPc, CuPc, AgPc, and PbPc are intermediate between these two. As Figure 5 shows, the amount of NO2 adsorbed irreversibly on the FePc mono(8) Buchholz, J. C. Appl. Surf. Sci. 1978, 1, 547. (9) Musser, M. E.; Dahlberg, S. C. Surf. Sci. 1980, 100, 605. (10) Sadaoka, Y.; Sakai, Y.; Yamazoe, N.; Seiyama, T. Denki Kagaku 1982, 50, 457.

Figure 7. Adsorption isotherms of NO2 on the FePc, CuPc, and ZnPc monolayers at 273 K. Open circles denote the first isotherms; closed circles denote the second isotherms. Table 2. Amounts of NO2 Adsorbed Irreversibly on the MPc Monolayers at 273 K central metal

V (mL (STP)/g)

central metal

V (mL (STP)/g)

H2 Mg AlCl VO Mn Fe Co

0.04 0.04 0.06 0.08 0.30 0.43 0.50

Ni Cu Zn Ag Sn Pb

0.09 0.13 0.02 0.13 0.36 0.20

layer is nearly proportional to the loading amount of FePc at loading amounts less than ∼2 × 10-5 mol/g. Its slope indicates the formation of stoichiometric compound, FePc‚ NO2 on graphite. Sadaoka et al.10 have reported the thermal desorption spectra of NO2 adsorbed on the powdered samples of H2Pc, CuPc, FePc, and CoPc. The first peak appeared in the temperature range 323-373 K for all the four substrates, whereas a second peak in the range 373-523 K was found only on CoPc and FePc. The first desorption peaks were ascribed to adsorbates bound weakly to the ligand parts of phthalocyanines, while the second peaks were ascribed to those bound strongly to the metal ions. Therefore, it may be reasonable to consider that the irreversible adsorption of NO2 observed here arises from the adsorption on the metal ions. Table 2 summarizes the amounts of NO2 adsorbed irreversibly at 273 K on the MPc monolayers. The amounts of the irreversible adsorbate on the MnPc, FePc, CoPc, and SnPc are roughly consistent with the monolayer capacity of MPc molecules on Grafoil graphite. In accordance with the thermal desorption spectroscopy studies, it is evident that NO2 is strongly bound to the metal ions of these monolayers to form stoichiometric compounds of composition MPc‚NO2 on graphite. NO2 is weakly bound to the metal ions of the VOPc, NiPc, CuPc, AgPc, and PbPc, and most of the adsorbate would be easy to desorb on evacuation at room temperature. Preferential adsorption of NO2 onto metal ions does not seem to occur with MgPc, AlClPc, and ZnPc. 3.5. NH3 Adsorption. Figure 8 shows the adsorption isotherms of NH3 on the MgPc, FePc, and NiPc monolayers at 273 K. Table 3 summarizes the saturated amounts of NH3 adsorbed at 273 K on the MPc monolayers, estimated from the Langmuir plot. MPcs can be classified into two classes according to the saturated amounts of NH3 adsorbed. In the cases of MgPc, MnPc, FePc, CoPc, and ZnPc, the saturated amounts of NH3 adsorbed are equal to or larger than 0.45 mL (STP)/g, namely, the adsorptive capacity needed for the formation of a stoichiometric compound MPc‚NH3 on graphite. In other cases, the saturated amounts of NH3 adsorbed are much smaller. Infrared studies11 concerning interaction of NH3 with the thin films of MnPc, FePc, CuPc, and ZnPc have shown (11) Steinbach, F.; Zobel, M. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2587.

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Morishige et al. Table 4. List of the MPcs Which Can Coordinate O2, NO2, and NH3 Molecules to the Metal Center in Their Monolayers to Form 1:1 Complexes O2 H2 Mg AlCl VO Mn Fe Co Ni Cu Zn Ag Sn Pb

Figure 8. Adsorption isotherms of NH3 on the MgPc, FePc, and NiPc monolayers at 273 K. Table 3. Saturated amounts of NH3 Adsorbed on the MPc Monolayers at 273 K central metal

V (mL (STP)/g)

central metal

V (mL (STP)/g)

H2 Mg AlCl VO Mn Fe Co

0.20 0.63 0.29 0.15 0.46 0.54 0.42

Ni Cu Zn Ag Sn Pb

0.22 0.24 0.61 0.28 0.24 0.28

that adduct formation is observed with FePc only. The stoichiometry of the adduct was shown to be FePc(NH3)2. This adduct is unstable in high vacuum at room temperature to become FePc. In these infrared studies, however, detection of surface species would have been impossible because of low surface area of MPcs evaporated onto KBr substrate. Uchida et al.12 have studied the interaction of pyridine with H2Pc and 3d metal Pcs by means of X-ray photoelectron spectroscopy and a gravimetric method. They have shown that adduct formation is observed with MnPc, FePc, CoPc, and ZnPc. Both pyridine and ammonia are Lewis bases. As pyridine is stronger in basicity than ammonia, the formation of coordination compound with the former may be easier than that with the latter and proceed into the inside of the MPcs microcrystals. Therefore, our results with NH3 adsorption are consistent with the previous results concerning the adduct formation of MPcs with pyridine. 3.6. Relationship between Adsorptive Activity and Central Metal. In the monolayers of square planar complexes of metal ions like MPcs, it is possible for the central metal to bind a ligand molecule to one of its two available axial coordination sites. Table 4 gives a list of the MPcs which can coordinate the gas molecules to the metal center in their monolayers to form 1:1 complexes on graphite. The kind of the central metals which exhibit adsorptive activity for O2 nearly overlaps with that for NO2. This is conceivable, because both O2 and NO2 are π acceptor ligands while NH3 is a σ donor ligand. The present experimental results clearly show that the d5 (12) Uchida, K.; Soma, M.; Onishi, T.; Tamaru, K. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2839.

NO2

NH3 O

O O

O O O

O O O O

O

O

Mn(II), d6 Fe(II), and d7 Co(II) in 3d metal complexes are active for both types of ligands. In these MPcs, the 3ddominated orbitals lie in the order 3dx2-y2 . 3dz2 > 3dxy ∼ 3dxz,yz and the dz2 orbital is only half filled.13 Therefore, these metal centers are good σ acceptors. In the case of the d8 Ni(II) and d9 Cu(II), the dz2 orbital is filled14 and is thus a poor σ acceptor. Studies15 of the O2 adducts of metalloporphyrins have shown that the key feature is that the metal has an unpaired electron in dz2 with high enough energy to spin pair with an odd electron in the oxygen antibonding orbital. In Co(II), this is the essential interaction binding the oxygen to the complex. In Fe(II) and Mn(II) complexes where there are unpaired electrons in metal orbitals with π-symmetry which permit interaction with the second unpaired electron in O2, spin-pairing of these electrons can also occur. Therefore, the O2 adducts of Fe(II) and Mn(II) complexes are more stable than that of Co(II) complex. Similar consideration may be extended to the O2 adducts of the MPcs. The d0 Mg(II) and d10 Zn(II), which have nearly the same effective size for the same coordination number, exhibit similar Lewis acidity toward σ donor ligands.16 It is impossible for the monolayers of five-coordinate AlClPc and VOPc to bind a molecule to its metal center, because the axial coordination site available to a ligand molecule is already occupied. More accurate and detailed explanations concerning the relationship between the adsorptive activity toward various gas molecules and the central metal need further experimental studies in addition to theoretical studies. In conclusion, the present studies show that the ordered monolayers of a wide variety of metallophthalocyanines supported on graphite can be easily prepared by a simple method of equilibrium adsorption from gas phase at high temperatures. Adsorptive activity of the MPc monolayers is strongly dependent on the kind of the central metals, and all the MPc molecules constituting a monolayer are available for gas adsorption. Therefore, these monolayers are expected to show high activities toward the catalytic reactions in which MPcs are effective. In adsorption of CO on the FePc, the saturated amount of adsorption was suppressed below half of a monolayer capacity expected. The CO molecules bound to the Fe ion of the FePc molecules might affect the electronic state of the neighboring FePc molecules through energy-transfer due to overlap17 of the largely delocalized π-electrons with the electron cloud of the graphite. LA9702906 (13) Reynolds, P. A.; Figgis, B. N. Inorg. Chem. 1991, 30, 2294. (14) Rosa, A.; Baerends, E. J. Inorg. Chem. 1994, 33, 584. (15) Tovrog, B. S.; Kitko, D.; Drago, R. S. J. Am. Chem. Soc. 1976, 98, 5144. (16) Collins, D. M.; Hoard, J. L. J. Am. Chem. Soc. 1970, 92, 3761. (17) Eastwood, D.; Lidberg, R. L.; Dresselhaus, M. S. Chem. Mater. 1994, 6, 211.